3-D structures with smoothly-varying topographical features in photo-sensitive epoxy resists

ABSTRACT

3-D structures which are fabricated by gray-tone exposure of a class of thick negative photo-sensitized epoxy resists from the substrate side of a transparent substrate, using development methods that rely upon a physical distinction between polymerized (solid) and unpolymerized (liquid) photoresist at elevated temperatures Such structures may exhibit smoothly-varying topographic features with thicknesses as great as 2 mm.

PRIOR APPLICATION

This is a division of prior application Ser. No. 09/902,029, filed onJul. 10, 2001 now U.S. Pat. No. 6,485,306, the priority of which ishereby claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to techniques for processing photoresistmaterials generally used in the fabrication of microelectronic,micro-optical and micromechanical devices.

2. Description of the Background Art

Various types of microelectronic, micro-optical, and/or micromechanicalstructures may exhibit overall dimensions ranging between 50 microns(μm) to a few millimeters (mm). Such structures are useful in creatingdevices that are purely mechanical (such as watch gears),electro-mechanical (such as electrostatically-driven vibratingelements), optical (such as arrays of lenses), electro-optical (such asmovable micro-mirrors), or for use with fluids (such as ink-jet printingheads).

A common method for fabricating such structures utilizes positivephotoresist, which is applied in a thin layer to a substrate. Followingapplication to the substrate, the free surface of the positivephotoresist is positioned under a mask that is opaque in some regionsand transparent in other regions. The positive photoresist issubsequently exposed on its free surface to ultraviolet (UV) light,which is patterned by passage through the mask. Positive photoresist issoftened by exposure to UV light, and when the exposed photoresist layeris subsequently developed by rinsing in a developing solution, theUV-exposed and softened regions dissolve in the developer and wash away,leaving the unexposed photoresist in place on the substrate.

Typically, the positive photoresist layer is less than 100 μm inthickness, and in most case, the entire thickness of this layer in theUV-exposed regions is removed by the developer, yielding photoresiststructures that have nearly vertical side walls. It is typical that theside of the substrate upon which the photoresist resides is thensubjected to an etching process that transfers the pattern in thephotoresist to the underlying substrate. The substrate is shielded fromthe etching process in the regions in which photoresist not exposed tothe UV light still remains on the substrate. By this sequence of steps,and by repeated application of this sequence of steps, structures withcomplex shapes may be fashioned in the substrate. One may use thissequence of steps to fabricate micro-structures having minutely steppedsurfaces, for example.

If a continuously curved photoresist structure is required, one mayexpose positive photoresist on its free surface through a mask thatpermits varying doses of UV light to penetrate the mask and illuminatethe photoresist. The mask may be a gray-tone mask, in which differentareas contain different UV light transmission fractions; or it may be amask having very small low UV transmission dots of varying sizes or dotdensities, selectively placed in regions of otherwise high UV lighttransmission. Regions of the positive photoresist exposed to a highenough dose of UV light will soften throughout the thickness of thephotoresist, as described above. Regions of photoresist exposed tolesser doses will soften from the exposed free surface to diminisheddepths, depending on the UV light dose. Upon development, the positivephotoresist remaining on the substrate will exhibit variations inthickness, corresponding to the variations in UV dose that the maskallowed to penetrate to the photoresist layer. This pattern of varyingthickness of positive photoresist can then be transferred to thesubstrate by known dry etching techniques.

A major limitation results from the fact that the greatest thickness ofpositive photoresist that may be processed in the manner described aboveis generally less than 100 μm. This is a result of the high absorptionof UV light in positive photoresist. If one attempts to expose too thicka layer, the UV light fails to adequately penetrate the deeper-lyingvolumes of the positive photoresist. As a result, the deeper-lyingvolumes fail to soften, therefore undesirably preventing full-thicknessremoval of the resist.

For some applications, however, it is advantageous to fabricate orfashion photoresist structures having thicknesses that are much greaterthan those attainable using positive photoresist.

A very useful class of photo-sensitized epoxy resists has been developedwhich has been shown to be useful at resist thicknesses up to 2 mm. Anexample of this class of resists is SU-8, currently manufactured byMicroChem Corp. of Newton, Mass., and by Sotec Microsystems SA, ofRenens, Switzerland. This resist is a negative photoresist; in contrastto the behavior of positive photoresist, SU-8 toughens by polymerizationupon proper exposure to UV light (of wavelengths near 365 nm). Anattractive feature of this class of materials is its ability to producestructures with almost vertical side walls with thicknesses as great as2 mm, which is much greater than that of any other photoresist.

All available literature that discusses the processing of this materialis directed at teaching the best sequence of steps and parameters ofindividual process steps to improve the user's ability to fabricatestructures in the photoresist having the full height of the film andnearly vertical side walls. Although the material can be spun on asubstrate at a variety of initial thicknesses, all structures thenfabricated in that layer are generally expected to have essentially thesame thickness as that of the initial film.

In order to achieve structures in the photoresist having varyingthicknesses, a step approximation to smoothly curved surfaces can befabricated using a technique in which several layers of the photoresistare applied in succession, each one being processed individually. Thistechnique can be used for fabricating a final structure containingsteps, each step corresponding to the thickness of one of the severallayers making up the final structure. In this technique, again, onefinds that the steps are nearly vertical.

A well-accepted sequence of process steps for fabricating structureshaving the full height of the photoresist film with nearly-vertical sidewalls in a layer of SU-8 is as follows:

-   1. Clean the substrate and apply an adhesion-promoter like    hexamethyidisilazane (HMDS).

Subsequent steps are done in a room in which green, blue and UV lightare excluded (orange room), since SU-8 is sensitive to short wavelengthvisible and near ultraviolet light.

-   2. Spin-on a desired thickness of SU-8. The starting material is a    viscous liquid mixture of SU-8 resin (typically bisphenol A novolac    glycidyl ether), a solvent for SU-8 such as γ-Butyrolactone (GBL) or    propylene glycol methyl ether acetate (PGMEA), and a photo-acid    generator such as a triaryl sulfonium salt (e.g. Cyracure UVI, Union    Carbide Corp.). Varying the ratio of resin to solvent yields    mixtures with different viscosities at room temperature. One selects    the mixture that will permit spinning the layer of desired thickness    at spin speeds between approximately 500 and 5000 rpm.-   3. The substrate with its spun-on layer is permitted to rest on a    level surface so that the viscous SU-8 layer can flatten, and then    the substrate is placed on a hotplate with final temperatures in the    neighborhood of 950 C (softbake). The step causes evaporation of the    solvent from the layer. As the solvent evaporates, the SU-8 layer    that remains on the substrate becomes more viscous. However, it    remains a liquid at 95° C. since even pure SU-8 has a glass    transition temperature of approximately 55° C.-   4. When the solvent has evaporated from the spun-on layer, the    substrate is removed from the hotplate and cooled to room    temperature. At room temperature, the SU-8 layer is a solid.-   5. The SU-8 layer is exposed with light whose wavelengths are    between 300 and 400 nm. The light is patterned on the SU-8 layer by    the use of a mask which has areas that are opaque to the exposing    illumination as well as areas that are transparent. This process is    typically carried out in a mask aligner, and the mask may be first    positioned to align with structures already on the substrate. In    this step, the exposure is made from the SU-8 side of the substrate.    The areas of the SU-8 that are exposed to this light release    photo-acid from the photo-acid generator which causes the SU-8 resin    to crosslink. An important consideration in selecting the energy per    area (dose) of the exposure is to assure that the entire film    thickness of exposed areas will polymerize completely.-   6. The substrate with its exposed SU-8 layer is placed on a    hotplate, with final temperatures of at least 95° C. This step    greatly accelerates the cross-linking of the SU-8 material in the    areas exposed to the UV light. In the ideal case, the entire    thickness of SU-8 material in exposed areas will become fully    polymerized. The material in the unexposed areas remains    unpolymerized.-   7. The patterned SU-8 layer is developed. The standard method    involves placing the substrate with its exposed SU-8 film in a bath    or in a sequence of baths containing a solvent for unpolymerized    SU-8. In these baths, the unexposed and unpolymerized areas of the    SU-8 film are dissolved away and ideally only the polymerized areas    remain attached to the substrate.-   8. The substrate with its developed fully-polymerized SU-8    structures is then rinsed and dried.

Methods for creating structures with smoothly-varying thicknesses inthis class of materials do not exist. If processes designed for creatingcontinuously curved surfaces in positive photoresist were applied tothis material, such as by exposing the SU-8 on its free surface througha mask that permits varying doses of UV light to penetrate the mask andexpose the photoresist, the SU-8 would polymerize first near the freesurface if the dose there were sufficient., At greater depths into thephotoresist film, the SU-8 might not polymerize. With development, theunpolymerized volumes of SU-8 near the photoresist-substrate interfacewould dissolve, and the entire SU-8 film would undesirably lift off thesubstrate.

What is needed is a method for fabricating structures in this andrelated classes of materials, where such structures may be characterizedby thicknesses that vary smoothly with position. The method shouldminimize the number of masking and other processing sequences requiredto fabricate such structures using a film of suitable initial thickness.

SUMMARY OF THE INVENTION

The present invention comprises 3-D structures with smoothly-varyingtopographic features and continuously-varying thickness which can befabricated by using one layer of photo-sensitized epoxy resist and a oneexposure and development process sequence. The structures are formed ina layer of photo-sensitized epoxy resist having an initial selectedthickness residing on a substrate, by exposing said layer, comprising amaterial such as SU-B, to doses of UV light which impinge on said layerfrom the substrate side, said doses varying across the width and breadthof the surface of said substrate.

In one embodiment, the substrate is transparent or essentiallytransparent to said light. Thus, appropriate substrates include mostglasses, fused silica, and a variety of polymers. As said lightpropagates through said layer, the intensity of said light decreasesmonotonically due to the absorption of said light by chemical componentsof said layer. We utilize a known property of photo-sensitized epoxyresists that requires that a minimum exposure dose, which is defined asthe product of the intensity of said light times the duration of theexposure, is necessary to polymerize said photo-sensitized epoxy resist.Due to said decrease in said light intensity, the exposure dose alsodecreases monotonically in the direction of exposure light propagationwithin said layer. Thus, by suitable selection of impinging UV lightintensity and exposure duration, one may cause a portion of said layerto polymerize, wherein said polymerized portion has thickness less thanor equal to said initial thickness, and resides adjacent to thesubstrate. By varying the intensity of said impinging exposing lightacross the width and breadth of the surface of said substrate, thethickness of the polymerized portion of the layer can be made to varyfrom zero thickness up to the initial thickness of the layer.

The unpolymerized portion of the layer can be removed from the substrateby heating the layer to a temperature in excess of the glass transitiontemperature of said unpolymerized portion, at which temperature thepolymerized portion is a solid and the unpolymerized portion is aliquid, and causing the unpolymerized portion to flow off the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a scanning laser exposure system that maybe used to selectively expose regions within a photo-sensitized epoxylayer in accordance with the present invention.

FIG. 2A is a perspective illustration of a substrate upon which aphoto-sensitized epoxy resist layer resides, and an exemplary exposurethereof in accordance with an embodiment of the invention.

FIG. 2B is a perspective illustration of an exemplary latent imagewithin the photo-sensitized epoxy resist layer of FIG. 2A.

FIG. 2C is a perspective illustration of an exemplary smoothly varyingthree-dimensional resist profile created in accordance with anembodiment of the invention.

FIG. 3A is a perspective illustration of a gray-tone mask upon which aphoto-sensitized epoxy resist layer resides, and an exemplary exposurethereof in accordance with an embodiment of the invention.

FIG. 3B is a perspective illustration of an exemplary latent imagewithin the photo-sensitized epoxy resist layer of FIG. 3A.

FIG. 3C is a perspective illustration of an exemplary smoothly varyingthree-dimensional resist profile created in accordance with anotherembodiment of the invention.

FIG. 3D is a perspective illustration of a gray-tone mask having aphoto-sensitized epoxy layer thereupon, and positioned for exposure inaccordance with a conventional exposure tool.

FIG. 4 is an exemplary semilog graph of polymerized SU-8 layer thicknessversus exposure dose.

FIG. 5 is a perspective view of a fiber positioning structure designedand/or fabricated in accordance with the present invention.

FIG. 6 is a gray-tone image of a gray-tone mask that may be used toproduce the optical fiber positioning structure of FIG. 5.

DETAILED DESCRIPTION

The present invention comprises a new process by which 3-D structureswith smoothly-varying topographic features and continuously-varyingthickness can be fabricated using one layer of SU-8, one mask, and oneexposure and development process sequence. In one embodiment, thesubstrate is transparent or essentially transparent to the light neededfor exposure of the SU-8. Thus, appropriate substrates include mostglasses, quartz, and a variety of polymers.

Process Sequence

The following processing steps may be performed in accordance with oneembodiment of the invention:

-   1. Clean a substrate and apply an adhesion-promoter, if desired    and/or necessary.    Subsequent steps may be performed in an orange room.-   2. Spin-on or otherwise apply a layer of SU-8 at least as thick as    the thickest structure desired. For some applications, the thickness    of the applied SU-8 layer may be 10%-20% thicker than the thickness    of the thickest structure desired (or possibly even thicker). In    other applications, the thickness of the applied SU-8 layer may    exactly or essentially exactly match that of the thickest structure    desired.-   3. Level the spun-on film at room temperature, and/or softbake the    film in a manner analogous to that previously described.-   4. Remove the substrate from the hotplate and allow it to cool to    room temperature.-   5. Expose the SU-8 layer from the substrate side using light    characterized by wavelengths between 300 and 400 nm (i.e., light    characterized by a wavelength, wavelength range, or spectral content    that may result in appropriate polymerization or cross-linking    within the SU-8 layer). The light may be patterned by the use of a    gray-tone mask, which may include areas of varying opacity relative    to the exposing illumination, in a manner readily understood by    those skilled in the art. A gray-tone mask may be designed and/or    fabricated, for example, in the manner described in U.S. Pat. No.    5,310,623, entitled “Method for Fabricating Microlenses,” by Gal.    Exposure may be carried out on a mask aligner, and the mask may be    first positioned to align with structures already on the substrate.

Other methods for exposing areas of the SU-8 layer with different dosesof exposing illumination, such as via a laser scanning device, are alsopossible. FIG. 1 is a block diagram of a scanning laser exposure system5 that may be used to selectively expose regions within aphoto-sensitized epoxy layer 110 such as SU-8 in accordance with thepresent invention. The scanning laser exposure system 5 may comprise alaser 10, a modulator 20, a focusing element 30, a deflection apparatus40; a substrate carrier 50, and a control computer 60. The substratecarrier 50 holds a substrate 100 upon which the photo-sensitized epoxylayer 110 resides.

The laser 10 generates light having a spectral content capable ofcausing the polymerization of the photo-sensitized epoxy layer 110. Theintensity of the laser light may be modulated 20 under computer control,passed through a focusing lens which may focus the light within thephoto-sensitized epoxy layer 110 and then to the deflection apparatus40. The deflection apparatus 40 may comprise a moving mirror, whichreflects laser light through the substrate 100 and into thephoto-sensitized epoxy layer 110 under the direction of the computer 60.The deflection apparatus 40 may be designed to deflect or reflectmodulated and focused laser light onto a given plane transverse to theplane of the substrate within the photo-sensitized epoxy layer 110. Insuch a situation, the substrate carrier 50 may adjust the position ofthe substrate 100 and the photo-sensitized epoxy layer 110 along anydirection to effectuate particular exposures and exposure patterns. Thesubstrate carrier 50 may perform position adjustment in response tosignals received from the computer 60.

Regardless of the particular type of exposure system employed, the areasof the SU-8 that are exposed to a greater light dose will cross-linkfrom the substrate interface up through a greater film thickness thanareas that are exposed to a lesser dose. An important consideration indesigning the gray-tone mask, or any other variable-exposure means forcontrolling the local dose of the exposure, is a measured characteristicof the SU-8 that describes how the thickness of SU-8 that willpolymerize as a function of exposure dose for the light used to exposethe film.

-   6. Place the substrate with its exposed SU-8 layer on a hotplate,    with final temperatures of at least 95° C. This step greatly    accelerates the cross-linking of the SU-8 material in the areas    exposed to the UV light. However, since the SU-8 has a known    absorption length for the light used in the exposure step, regions    of the exposed film nearest the substrate will receive higher doses    than regions further from the substrate. For each formulation of    SU-8, there exists a minimum exposure dose that is required to    effect sufficient cross-linking such that the material so exposed    will be a solid at 95° C. Regions of the SU-8 film which do not    receive this minimum exposure dose will remain unpolymerized, and    therefore liquid, at 95° C.-   7. Develop the patterned SU-8 layer. Although solvent development    techniques similar to the process described above may be effective    is removing the unexposed SU-8 photoresist, such solvent development    techniques tend to leave the surface of the remaining polymerized    SU-8 film rough (with surface roughness of as much as 2 μm rms). The    remaining polymerized SU-8 structures also tend to absorb solvent    and swell, thus distorting their size and shape. The swelling can    undesirably increase the volume of polymerized structures by 30% or    more, and can leave them permanently deformed, even after the    solvent has been fully removed from the remaining polymerized    structures. The solvent swelling can also cause SU-8 structures to    lift off the substrate due of the strain at the interface between    the substrate and the swelled SU-8.

Solvent swelling may be a challenge in this process because thepolymerized SU-8 is weakly cross-linked. It is polymerized justsufficiently to be solid at 95° C. Typically, SU-8 manufacturersrecommend much higher doses of UV light to fully polymerize the SU-8layers. The polymerized SU-8 that results from exposure to higher dosesof UV light are so completely cross-linked that solvent development doesnot cause significant solvent swelling.

In the context of the present invention, a more suitable developmentprocedure may comprise a technique referred to herein as hot-flowdevelopment. Hot flow development relies upon a physical distinctionbetween polymerized (solid) and unpolymerized (liquid) photo-sensitizedepoxy at a given temperature. For SU-8, this temperature may be 95° C. Afirst variant of hot flow development is hot-spin development. In hotspin development, the substrate (upon which the exposed SU-8 layer orfilm resides) is placed on a spinner and heated to 95° C. The heatingmay be performed, for example, via a heated spinner chuck, blowingheated air onto the SU-8 film on the substrate while it is mounted onthe spinner, and/or irradiation with infrared light or some othereffective means.

When the SU-8 film reaches 95° C., the exposed and polymerized regionsof the film will be solid, but the regions of the film that have notbeen exposed to the minimum exposure dose required for polymerizationremain liquid. In many regions of the film, liquid unpolymerized SU-8may be lying above solid polymerized material. The heated wafer may bespun at rates as high as 7000 rpm. As a result, the liquid unpolymerizedSU-8 may be spun off the wafer by the centripetal forces, leaving only athin layer of liquid unpolymerized material still adhering to thesubstrate and/or to the underlying solid polymerized material. Thethickness of this thin adherent layer depends upon film viscosity andsurface tension forces at 95° C., but may be minimized by using higherrotational rates and longer spin times. The thickness of this film ofadherent unpolymerized SU-8 can be reduced to a few microns, although ittends to adhere in thicker volumes at concave corners in the polymerizedSU-8 structures.

At this point, a second variant of the hot-flow development proceduremay be employed. In this variant, the unexposed SU-8 still adherent tothe substrate and to the polymerized SU-8 structures may be furtherremoved by blowing heated gas at high velocity at the liquidunpolymerized SU-8 using a fine nozzle which creates a jet of heatedgas. In one embodiment, the liquid SU-8 may be propelled out of theconcave corners of the structures using this technique with a practicedhand while observing the process under a low-power stereo microscope.Alternatively, a motorized chuck upon which the substrate is held may bemanually, semi-automatically, or automatically positioned beneath such anozzle, where such positioning may be aided by a microscope or visionsystem, to facilitate liquid SU-8 removal.

In general, hot-flow development techniques or methods rely uponinherent differences in viscosity between polymerized and unpolymerizedSU-8 at or above 95° C. Those skilled in the art will understand thatthe present invention may additionally or alternatively employ multiplevariations of the hot-flow development method described above. SU-8films developed by hot-flow techniques or methods may exhibit surfaceroughness typically less than 200 nm rms, and do not suffer from solventswelling and/or delamination from the substrate that results from theexcessive strain produced by such swelling.

-   8. The heated substrate with its polymerized SU-8 3-D structures    with smoothly-varying topographic features is permitted to cool.

FIG. 2A is a perspective illustration of a substrate 100 upon which aphoto-sensitized epoxy resist layer 110 resides, and an exemplaryexposure thereof in accordance with an embodiment of the invention. InFIG. 2A, the resist layer 110 receives a given exposure dose at anappropriate wavelength of light through an exemplary gray-tone mask 200and the substrate 100. As indicated above, the substrate 100 may betransparent or essentially transparent to the exposing light.

During exposure, portions of the photo-sensitized epoxy resist layer 110may be cross-linked to varying degrees, in accordance with lightattenuation and/or patterning selectively performed via the gray-tonemask 200. FIG. 2B is a perspective illustration of an exemplary latentimage 150 within the photo-sensitized epoxy resist layer 110 of FIG. 2A.The latent image 150 corresponds to regions within the resist 110 thathave been exposed to at least a minimum dose necessary to causesufficient polymerization so that the volume of this latent image wouldbe solid at 95° C., and is surrounded by unpolymerized regions withinthe resist layer 110, in a manner readily understood by those skilled inthe art.

During development, unpolymerized material may be removed in mannersdescribed above, leaving polymerized material upon the substrate 100.FIG. 2C is a perspective illustration of an exemplary smoothly varying3-D epoxy resist profile 180 remaining upon the substrate 100 afterdevelopment, created in accordance with an embodiment of the invention.

In an alternate embodiment, a gray-tone mask 200 may serve as asubstrate or carrier upon which a layer 110 of photo-sensitized epoxyresist directly resides. That is, a separate substrate 100 of the typeshown in FIGS. 2A through 2C may not be required. FIG. 3A is aperspective illustration of a gray-tone mask 200 upon which aphoto-sensitized epoxy resist layer 110 resides, and an exemplaryexposure thereof in accordance with an embodiment of the invention.Relative to FIG. 2A, like reference numbers indicate like elements toaid understanding.

FIG. 3B is a perspective illustration of an exemplary latent image 160within the photo-sensitized epoxy resist layer 110 of FIG. 3A. Thelatent image 160 comprises regions within the resist layer 110 that havebeen selectively exposed to at least a minimum dose necessary to causesufficient polymerization so that the volume of this latent image wouldbe solid at 95° C., in accordance with light transmission through thegray-tone mask 200. Other regions within the resist layer 110 remainunpolymerized.

As above, development results in removal of unpolymerized material fromthe resist layer 110. FIG. 3C is a perspective illustration of anexemplary smoothly varying three-dimensional resist profile 190remaining after development, created in accordance with anotherembodiment of the invention.

Modern exposure tools typically include a chuck or platform upon which awafer or substrate may reside. A mask carrier typically resides abovethe chuck, and a light source is situated above the mask carrier. Insuch a conventional exposure arrangement, light travels from the lightsource, through a mask, through a resist layer, and to a substrate. Incontrast, in the present invention, light may travel from a lightsource, through a mask, optionally through a substrate, and into orthrough a photo-sensitized epoxy layer 110.

In one embodiment, the photo-sensitized epoxy layer 110 may itself bepositioned upon a chuck or platform, thereby facilitating exposure via aconventional exposure tool. FIG. 3D is a perspective illustration of agray-tone mask 200 having a photo-sensitized epoxy layer 110 thereupon,and positioned for exposure upon a wafer chuck 210 in accordance with aconventional exposure tool. In such an exposure situation, thephoto-sensitized epoxy layer 110 may contact the wafer chuck 210. Tominimize or eliminate undesirable surface damage and/or contamination,the photo-sensitized epoxy layer 110 may exhibit a thickness that isgreater than that of the thickest 3-D structure to remain followingdevelopment. Surface defects may be removed during development ifpolymerized resist does not extend throughout the entire thickness ofthe photo-sensitized epoxy layer 110.

In an alternate embodiment, in the event that a mask carrier (not shown)is present, a gray-tone mask 200 having a photo-sensitized epoxy layer110 thereupon may be positioned upon the mask carrier, and held in placevia a vacuum or other conventional technique. A wafer chuck 210 may belowered or positioned such that it does not contact the photo-sensitizedepoxy layer 110, thereby preventing the occurrence of such resistsurface layer defects.

Process Design Considerations

In order to design the gray-tone mask or other variable-exposure means,one must first know a resist characteristic that describes the thicknessof SU-8 that will polymerize as a function of exposure dose for thespecific light source used to expose the film. FIG. 4 is a graph showinga typical measured thickness versus dose characteristic for aconventional UV arc-lamp source, such as the UV source associated with aCanon PLA 501F contact mask aligner.

For a desired thickness of polymerized SU-8 at a chosen position on orwithin a layer, one may design the optical density of the gray-tone maskin conjunction with the known intensity of the illuminating lamp and theknown associated thickness versus dose characteristic, or otherwiseexpose the SU-8 layer such that the chosen area of the SU-8 filmreceives a light dose that will produce polymerization to a desiredthickness. By way of example, using a Canon PLA 501F contact maskaligner, which may have a measured exposing light intensity of 5.25mW/cm^2 in a wavelength band at 365 nm, an exposure time ofapproximately 170 seconds may be required to polymerize a 500 micronthickness of SU-8, when said exposing light first passes through aregion of a gray-tone mask having a transmissivity of approximately 70%.Extending this concept to an entire wafer leads to the design ofgray-tone masks or variable-intensity light scanning schemes.Manipulation of the spectral distribution of the light source may alsobe accomplished (using filters in the case of an arc-lamp light sourcein a mask aligner, or by choosing an appropriate laser source for alight scanning system) in order to adjust the thickness vs. dosecharacteristic of the SU-8 for particular applications.

FIG. 5 is a perspective view of an exemplary fiber positioning structure300 designed and/or fabricated in accordance with the present invention.The fiber positioning structure 300 may comprise a polymerized SU-8layer characterized by a curved V-groove having parabolically-shapedwalls, and which is 750 μm wide at the top, narrowing to a 125 μm widecylindrical slot at the bottom. The fiber positioning structure 300 is1.5 mm long. The fiber positioning structure 300 facilitates placementor positioning of the center of a 125 μm fiber at a distance of 200 μmabove a substrate surface.

FIG. 6 is a gray-tone image of a gray-tone mask 400 designed forproducing the fiber positioning structure 300 of FIG. 5. Exposure of anSU-8 layer using the gray-tone mask 400 and a uniform dose of, forexample, 900 mJoules/cm² may selectively polymerize appropriate regionswithin the SU-8 layer. Subsequent processing of the SU-8 layer inaccordance with process steps detailed above may produce the fiberpositioning structure 300.

Darker regions within the gray-tone mask 400, having reduced UVtransmission, cause a reduced dose to illuminate corresponding regionsof the SU-8 layer. These regions may polymerize to a reduced distancerelative to the substrate/photoresist interface. Lighter gray-tone maskregions, on the contrary, permit an increased dose to illuminatecorresponding regions of the SU-8 photoresist. These regions maypolymerize to an increased distance relative to thesubstrate/photoresist interface.

1. A product produced by the process comprising the steps of: applying alayer of photo-sensitized epoxy to the first side of a substrate havinga first and second side, wherein the layer of epoxy has a thickness ofat least 100 micrometers and the substrate is at least partiallytransparent to light to which the photo-sensitized epoxy is sensitive;exposing at least one region within the layer of photo-sensitized epoxywith light incident upon the second side of the substrate sufficient toproduce at least one region of polymerization of varying thicknesswithin the layer of photo-sensitized epoxy, such that upon removal ofthe non-polymerized regions of the layer, following polymerization ofthe one region, the resulting three-dimensional structure has acontinuously varying thickness and a smoothly varying topography, with athickness of at least 100 micrometers.
 2. A three-dimensional article,comprising: a substrate having a first side and a second side; aphoto-sensitized epoxy layer applied on the first side of the substrate,wherein the epoxy layer has a thickness of at least 100 micrometers andthe substrate is at least partially transparent to light to which thephoto-sensitized epoxy is sensitive, wherein the photo-sensitized epoxylayer has been exposed to light incident on the second side of thesubstrate sufficient to produce at least one region of polymerization ofvarying thickness within the layer of photo-sensitized epoxy, withnon-polymerized portions of the layer having been removed followingpolymerization of the one region, to produce a resultingthree-dimensional structure with a continuously varying thickness and asmoothly varying topography, and a thickness of at least 100micrometers.
 3. A three-dimensional article, comprising: a substratehaving a first side and a second side; a negative photo-sensitized epoxylayer applied on the first side of the substrate, wherein the layer ofepoxy has a thickness of at least 100 micrometers and wherein thesubstrate is at least partially transparent to light to which thephoto-sensitive epoxy is sensitive, and then processed, includingexposure of the photo-sensitive epoxy layer by light incident on thesecond side of the substrate, to produce at least one region ofpolymerization of varying thickness within the epoxy layer, resulting ina three-dimensional structure with a continuously varying thickness anda smoothly varying topography following removal of non-polymerizedregions of the epoxy layer, wherein the three-dimensional structure isat its thickest part at least 100 micrometers.
 4. An article of claim 3,wherein the three-dimensional structure is at its thickest part severaltimes thicker than 100 micrometers.